While you are under there, Daniel, check all around. Look for the obvious, such as an exhaust flange that is loose, a loose fuel evap cannister hose, etc...

Good luck!

You were right, Maddog! I finally got a chance to look under my truck today. It turns out that one of the "coupler" hoses going to the fuel emissions canister under the truck had a very nice crack in it. In less than 5 minutes I replaced that little rubber hose (the ones in between the non-repairable plastic emissions tubing) and the hissing was gone! Woohoo! The idle actually seems a bit higher too, which I guess makes sense!